Not Applicable.
This invention pertains to shielding apparatus for containing high frequency electromagnetic radiation within a personal computer, cellular telephone, or other electronic instrument. Electromagnetic compatibility (EMC) is a broad term used to describe electromagnetic interference (EMI), radio frequency interference (RFI) and electrostatic discharge (ESD), and the above terms are often used interchangeably.
Electronic devices are both sources and receptors of EMI which creates a two-fold problem. Since electromagnetic radiation penetrating the device may cause electronic failure, manufacturers need to protect the operational integrity of their products. Secondly, manufacturers must comply with the regulations aimed at reducing electromagnetic radiation emitted into the atmosphere. Proper design is necessary to prevent the device""s function from being disrupted by emissions from external sources and to minimize its system""s emissions.
The use of plastic as a housing material for electronic equipment has contributed to problems with EMI shielding because EMI waves pass freely through unshielded plastic without substantial impedance or resistance. Increasing clock speeds of microprocessors used in computing devices makes it more difficult to handle the EMI emission faster computers generate.
Current methods for shielding of electromagnetic interference (EMI) include the use of metal housings, metal filled polymer housings, metal liners for housings, and conductive coatings for the interior of rigid polymer or composite housings. Recent development in low mass shields are shown in U.S. Pat. No. 5,811,050 to Gabower. Shields described in that patent are commercially produced by Shielding for Electronics, Inc. of Sunnyvale, California.
Ever increasing clock speeds of personal computers being offered makes effective shielding more and more challenging since any gap in an EMI shield which has one dimension in excess of one-half wavelength may allow substantial EMI leakage, causing the unit to fail United States Federal Communication Commission standards.
The use of metallic coatings on rigid plastic housings presents certain manufacturing and service concerns. A slipped tool used during assembly or a repair can cause a scratch in the metal coating of sufficient size to cause a slot antenna, thereby making the case totally useless, and thereby leading to a costly item being discarded with little feasibility for successful recycling. The seams of a metal plated plastic housing will act like slot antennae unless the housing sections are conductively joined by the use of overlapping joints, conductive gaskets, or conductive tape. When the housing must be opened for a repair or retrofit, it can be understood that some of the conductive interconnection may be degraded by the activity of disassembly.
Further background on prior art methods and characteristics of shielding methods may be examined in xe2x80x9cEMI/RFI Shielding Guidexe2x80x9d published by the GE Plastics Division of the General Electric Company, in xe2x80x9cThe EDN Designer""s Guide to Electromagnetic Compatibilityxe2x80x9d by Gerke and Kimmel, Supplement to EDN Magazine, Volume 39, No. 2, (January, 1994) and in xe2x80x9cPlastics in Electronic Equipment Enclosures: Emerging Economic and Environmental Issuesxe2x80x9dby Peter Mooney, published by Plastic Custom Research Services, Adorance, N.C. (February, 1995).
The invention is a novel EMI Containment System for use in electronic equipment enclosures. A multi-compartmented plastic form preferably made of thermoformed plastic sheet, that is, sheet or film material which has been heated and drawn by air pressure into a mold or onto a die, is coated with a conductive metal coating. The metal coating may be applied by painting or preferably by vacuum metallization as described in U.S. Pat. No. 5,811,050 to Gabower. The resulting metallized form has a metal coating covering its surface to a thickness of at least one micron and the wall thickness of the plastic form is quite small, in the range of 0.003 to 0.020 inches, resulting in an inexpensive, nestable multi-compartment EMI shield for placement over elements mounted on a circuit board which emit electromagnetic radiation. The compartments of the form are arranged such that the form will conform to the inside shape of a housing for an electronic equipment such as a cellular phone, computer, or other device which internally generates EMI or which is susceptible to degradation if exposed to RFI from outside the device. The form is constructed with a laterally extending peripheral lip and with hollow walls which separate the compartments and which fit over interior ribs formed within the enclosure to which the shape of the form conforms.
Non-conductive elastomeric gasket material which may be applied as a liquid is inserted between the tops of the ribs and outer sidewall of the housing and under the hollow ribs and the peripheral lip of the form, thereby providing a cushion between the form and the housing.
A circuit board containing the EMI emitting elements is placed against the form such that the EMI emitting elements are received within the compartments. The circuit board is provided with a conductive ground trace on its surface facing the form, the ground trace circumscribing a path which coincides with the hollow ribs and peripheral lip of the form. Contact between the metallized form and the ground trace of the circuit board serves to enclose the EMI emitters within a grounded enclosure to contain the EMI within the unit and isolated from other elements within the unit. When the circuit board is mounted within the housing, the gasket forces the lip and hollow walls of the form into contact with the ground trace of the circuit board.
The lip and hollow walls of the form may alternatively be formed with closely spaced dimples, puncture protrusions, or extending tabs, which are coated with metal and extend from the form to increase contact between the form and the ground trace of the circuit board, the spacing of the gap-filling dimples, puncture protrusions or extending tabs being selected to prevent spaces between them from acting as slot antennae. The elastomeric gasket may be omitted when the alternate embodiment form is used.
It is an object of the invention to provide an EMI containment apparatus which does not require. conductive gaskets to assure creation of a grounded enclosure around EMI emitting components.
It is another object of the invention to provide an inexpensive EMI shield system which permits shielding of EMI emitting components of an electronic device from susceptible components of the same device.
It is a further object of the invention to provide an inexpensive, lightweight EMI Shielding system.
It is a further object of the invention to provide an EMI containment system which provides conductive contact with a ground trace formed on the surface of a circuit board containing EMI emitting components.
It is another object of the invention to provide an EMI shield which may be shaped to conform to the internal ribbing of a rigid enclosure which needs no gasket element to create contact with a surfaced ground trace of a circuit board.
These and other objects of the invention will become apparent from examination of the description and claims which follow.